So one of the most predominant views in America of this time was the idea of the Yellow Peril. Perhaps most popularized by the character "Fu Manchu," it was thought that the "Yellow Races"-primarily Chinese, but also Japanese-would grow in power and overthrow the white man's rule. There were many anti-Chinese and Japanese laws, the most well-known ones being the Chinese Exclusion Act and the so-called Gentleman's agreement between Japan and the US that resulted in the barring of Chinese and Japanese immigration to the US, respectively.

That being said, despite the anti-Asian rhetoric (which also differed, with areas like the deep South being more or less ambivalent about Asians, while California was one of the most vitriolic) there was local support for the Chinese. The so-called China Lobby was one of the most powerful lobbies in the US at this time, equivalent to the modern day Israel Lobby. These people supported China, and especially Chiang Kai-Shek. They included missionaries (who saw Chiang, being a Methodist, as a new Christian leader for China), businessmen (who saw the great market of China as being the future market of American goods), and of course the Chinese (various Kuomintang representatives, which later would include members like TV Soong, head of the Chinese Development Financial Corporation, and Soong Mei-ling, the wife of Chiang Kai Shek). The end result was that there was quite a powerful political force that caused US government officials, including Roosevelt, to take a pro-China view.

There were several incidents involving Americans in China that caused the media to pay attention. For instance, the battle of Shanghai was well-covered by foreign journalists, who often were able to record the battle scenes from the various foreign concessions that were not involved in the battle. There were stories of for instance air raids and stray artillery that would hit near foreign districts of the city. Generally coverage favored the Chinese. Later, the bombing of the USS Panay by Japanese aircraft created more of a media uproar, more so because American newspapermen were on the Panay when it was attacked. Japan publicly apologized but the news coverage was already quite pro-China (although notes of Japanese atrocities was not as prominent-they were certainly mentioned but details were spotty).

By the time the US finally got involved in the war with Japan's bombing of Pearl Harbor, US aid was already flowing to China and a large amount of the populace felt that the US should intervene against Japan. The actual bombing of course turned the jingoism machine up to 11, and you had articles like this Life piece which tried to tell people how to differentiate between Chinese and Japanese people using gross racial stereotypes (which often were a reversal of comparisons between the Chinese and Japanese just a few years before). This did cause the China theater to rise to the top of American screens for a time, though. Notably, the Battle of Changsha, where the KMT repulsed a large Japanese offensive, became acclaimed as the first victory against Japan amongst the Allied powers.

Sources:

Chang, The Chinese in America

Harmsen, Shanghai 1937: Stalingrad on the Yangtze